MB39A123 Datasheet, PDF (23/44 Pages) Fujitsu Component Limited. – 6 ch DC/DC Converter IC with Synchronous Rectification

MB39A123
■ SETTING THE SOFT-START TIME
To prevent rush currents when the IC is turned on, you can set a soft-start by connecting soft-start capacitors
(CS1 to CS6) to the CS1 terminal (pin 39) to CS6 terminal (pin 27) respectively.
As illustrated below, when each CTLX is set to “H” from “L”, the soft-start capacitors (CS1 to CS6) externally
connected to the CS1 to CS6 terminals are charged at about 1.1 µA.
The error amplifier output (FB1 to FB6) is determined by comparison between the lower voltage of the two non-
inverted input terminal voltage (1.23 V (ch.1 : 1.0 V) , CS terminal voltage) and the inverted input terminal voltage
(−INE1 to −INE6) . The FB terminal voltage is decided for the soft-start period (CS terminal voltage < 1.23 V
(ch.1 : 1.0 V) ) by the comparison between −INE terminal voltage and CS terminal voltage. The DC/DC converter
output voltage rises in proportion to the CS terminal voltage as the soft-start capacitor externally connected to
the CS terminal is charged. The soft-start time is obtained from the following formula :
Soft-start time : ts (time until output voltage 100%)
ch.1
: ts (s) =: 0.91 × CS1 (µF)
ch.2 to ch.6 : ts (s) =: 1.12 × CSX (µF) X : Each channel number
Vo
R1
−INEX
R2
CSX
CSX
FBX
CTLX
VREF
1.1 µA
L priority
−
+
+
Error
AmpX
1.23 V (ch.1 : 1.0 V)
CHCTL
H : CSX can be charged when CTLX is set to "H" and
normal operation is selected
L : CSX is discharged when CTLX is set to "L" and
protective operation is selected
X : Each channel number
23